“What you need to do is to increase your total body fluids another way,” Dr. Martin said.

He added that the legal, safe way to do it is through glycerin loading. The technique exploits the unusual properties of glycerin, a thick, gooey sugar alcohol that is sold in drugstores as a lubricant. Each molecule of glycerin absorbs three molecules of water. During a race, the body uses the glycerin for energy. And every time the body metabolizes a molecule of glycerin, “it unleashes three molecules of water,” Dr. Martin said.

The result, he said, is that “you have a water bank account.”

Glycerin loading, he added, should be reserved for races of a half marathon or longer, when runners are competing in intense heat for at least an hour and a half.